In recent years, considerable research and development works have been undertaken to produce various electric vehicles propelled with electric vehicle motors instead of engines. As a typical example of such an electric vehicle, a development work has rapidly been proceeded to provide a fuel cell powered vehicle which employs a proton exchange membrane type fuel cell (hereinafter referred to as PEM type fuel cell) which is abbreviated as, for example, PEMFC (Proton Exchange Membrane Fuel Cell).
The PEM type fuel cell is constructed of a stack whose structure is composed of a plurality of unit cells each for generating electric power output. Each of the unit cells is constructed having an anode side separator formed with a hydrogen gas flow passage, a cathode side separator formed with an oxygen flow passage, and a membrane and electrode joint member, which is sandwiched between two separators and which is abbreviated as MEA (Membrane Electrode Assembly). One surface of the proton exchange membrane of the MEA carries thereon an anode side electrode catalyst layer and a gas dispersion layer which are sequentially stacked, and the other side of the proton exchange membrane carries a cathode side electrode catalyst layer and a dispersion layer which are sequentially stacked.
In such a PEM type fuel cell, the presence of a humid condition of the MEA allows hydrogen ion to pass through the proton exchange membrane to the cathode side to cause each unit cell to generate an electromotive force of about 1 V. Further, the PEM type fuel cell is arranged to produce the most stable power output in a temperature circumference of, for example, about 75 to 85° C., with a specific electric circuitry being configured to propel the electric vehicle motor through a power drive unit and a power output current control unit.
It has been a usual practice to have the fuel cell powered vehicle, on which the PEM type fuel cell is mounted, equipped with a cooling system for maintaining the fuel cell within a given temperature range. For example, U.S. Pat. No. 5,605,770 discloses a cooling system which comprises a primary cooling circuit for circulating primary coolant to cool the fuel cell, and a secondary cooling circuit for circulating secondary coolant which can be heat exchanged with the primary coolant and which is circulated through a radiator of the vehicle. In such a fuel cell powered vehicle, also, a cooling system is located in the vehicle for cooling a heat generating source composed of the fuel cell and the electric vehicle motor. For example, Japanese Laid-open Patent Publication No. 11-178116 discloses a cooling system for cooling the fuel cell and the vehicle propelling motor (electric vehicle motor) with coolant water which circulates through the radiator equipped with an electric motor driven fan.
In the cooling system for the prior art fuel cell powered vehicle disclosed in the above US patent, a circulation pump, which circulates primary coolant in the primary cooling circuit, and a circulation pump, which circulates the secondary coolant in the secondary cooling circuit, are mounted in the vehicle independently of each other. As a consequence, these circulation pumps require two discrete electric motors. Also, in order to control the amount of the primary coolant to be circulated and the amount of the secondary coolant to be circulated to optimum values for thereby maintaining the fuel cell to a suitable temperature range, a complicated, cooperative control is required to control the two discrete electric motors in a mutually operating relationship.
On the other hand, in the cooling system disclosed in the above Japanese Patent Publication No. 11-178116, the presence of the flow of coolant water allows for the fuel cell, a compressor, a fuel cell system, which includes a reformer, a power converter and a vehicle propelling motor (the electric vehicle motor) to be cooled. As a consequence, the aforementioned radiator is required to have a large cooling capacity, with a resultant increase in the size of the radiator. Further, a problem is encountered in the prior art cooling system in that it is difficult to cool the fuel cell and the electric vehicle motor to respective optimum temperatures. Although such a problem can be solved by separately locating the cooling system for the fuel cell and the cooling system for the electric vehicle motor etc. to be completely independent from one another, a flow path of the cooling system suffers from an unduly extended length.